Power MOSFETs and IGBTs are frequently employed to drive an inductive load, such as an ignition coil. Due to this type of load, overvoltages appear across the device when it is switched off; said overvoltages can be very high, stressing the device as it drains the nominal current for a short time interval after it has been switched off. Moreover, high overvoltages can lead to the breakdown of the device, and to its permanent damage if it is not able to dissipate the energy stored in the inductive load.
To prevent such problems, power devices are generally protected with an external clamping diode having a breakdown voltage lower than that of the device to be protected. Stray inductances and operating time of the protection device, however, limit the effectiveness of such protections.
In order to overcome such limits, the manufacturers of power MOS devices have developed integrated structure protection circuits, called "active clamps", comprising a resistive element, connected in series between an external gate electrode and the gate of the power device, and a clamping element between the gate and the drain of power device. When the power device is switched off, overvoltages give rise to a voltage drop across the terminals of the clamping element; if said voltage drop exceeds the breakdown voltage of the clamping element, current will flow through it and the resistive element, between the drain of the power device and the gate electrode. As soon as the voltage drop across the resistive element exceeds the turn-on voltage of the power device (which, in the case of the power MOSFETs is represented by the threshold voltage), this will turn-on again and start draining current; thus the current that must be sustained by the active clamp is very low. Overvoltages across the power device will be clamped to a maximum value (Vclamp) represented by the sum of the breakdown voltage of the clamping element and the turn-on voltage necessary to sustain the current flowing through the power device itself.
It should be understood that a simple diode, whose anode and cathode are respectively connected to the gate and to the drain of a n.sup.- channel power MOSFET, is not suitable. This is because when the power MOSFET is in the on state, the diode would be forward biased, and the current flowing from the gate to the drain through said diode would introduce an offset equal to the forward voltage of the diode in the output characteristics of the device.
A possible clamping element is represented by a pair of diodes back-connected by their anodes, while their cathodes are respectively connected to the gate and the drain of the power MOSFET.
It is furthermore possible to serially connect a plurality of such diodes, provided that the gate of the power MOSFET is not connected to any of their anodes: in this way it is possible to reach the desired value for Vclamp.
In the prior art mentioned in U.S. Pat. No. 5,162,966, an active clamp for a power MOSFET is described wherein the clamping element utilizes a chain of n.sup.+ /p.sup.+ junction diodes. The anode of each diode is made up of a p.sup.+ region extending from the semiconductor surface into an n epitaxial layer representing the drain of the power MOSFET, while the cathode is an n.sup.+ diffusion into said p.sup.+ region.
Each diode of the chain has associated with it a parasitic diode whose anode and cathode are respectively represented by the p.sup.+ region and by the n epitaxial layer. It follows that the gate of the power MOSFET shall not be connected to any diode anode, in order to prevent one of said parasitic diodes from affecting the output characteristics of the device in the on state, because of the electrical connection between the gate and the drain. To this purpose, in the described structure the first diode of the chain is connected to the gate of the n-channel power MOSFET by its cathode, and has its anode connected to that of the second diode of the chain. All of the other diodes have their cathodes connected to the anode of the following diode, while the cathode of the last diode is connected to the drain of the power MOSFET.
A problem arising from the use of p.sup.+ /n.sup.+ junction diodes is represented by their low breakdown voltage, typically in the range of a few volts. When the power MOSFET is in the on state, the first diode of the chain is reverse biased; if the voltage across if exceeds its breakdown, current will flow through it and its associated parasitic diode, thus between the gate and the drain of the power MOSFET, affecting the output characteristics of the power device.
Another problem associated with the low breakdown voltage of the p.sup.+ /n.sup.+ diodes of the chain is the necessity of a series connection of a large number of the diodes in order to operate with a high Vclamp value; this causes complications on the layout and increases the area occupied by the protection circuit.
A further problem is represented by the impossibility to obtain active clamps with a small thermal drift coefficient, since this parameter is given by the sum of the thermal coefficients of the breakdown voltage of each diode of the chain.
Finally, it is impossible to obtain active clamps with Vclamp values different from the sum of the threshold voltage of the power MOSFET plus an integer multiple of the breakdown voltage of a p.sup.+ /n.sup.+ diode.